1. Field
The present invention relates generally to data communication, and more specifically to techniques for scheduling data transmission to terminals with variable scheduling delays.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users, and may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques. CDMA systems may provide certain advantages over other types of system, including increased system capacity.
Many CDMA systems are capable of supporting different types of services (e.g., voice, packet data, and so on) on the forward and reverse links. Each type of service is normally characterized by a particular set of requirements. For example, voice service typically requires a fixed and common grade of service (GOS) for all users as well as a relatively stringent and fixed delay. In particular, the overall one-way delay of speech frames may be specified to be less than 100 msec. These requirements may be satisfied by providing a fixed and guaranteed data rate for each user (e.g., via a dedicated traffic or code channel assigned to the user for the duration of a communication session) and ensuring a particular maximum error rate for speech frames independent of the link resources. To maintain the required error rate at a given data rate, a higher allocation of resources is required for a user having a degraded link.
In contrast, packet data service may be able to tolerate different GOS for different users and may further be able to tolerate variable amounts of delays. The GOS of the packet data service may be defined as the total delay incurred in the transfer of a data message. This delay can be a parameter used to optimize the efficiency of a data communication system.
In a wireless communication system, the total transmit power available at any given base station is typically fixed. For a multiple-access communication system, the total transmit power may be allocated and used to transmit data to a number of terminals concurrently. At any given moment, multiple terminals having different requirements may desire data transmission. To support both types of service (i.e., voice and packet data), a CDMA system can be designed and operated to first allocate transmit power to voice users requiring a particular GOS and shorter delays. Any remaining transmit power can then be allocated to packet data users whom can generally tolerate longer delays.
A key goal of a wireless communication system is then to schedule the data transmission for packet data users such that high system performance is attained. Numerous challenges are encountered in scheduling data transmission to the terminals for these packet data users. First, the amount of transmit power required by voice users can change from frame to frame, which would then correspondingly affect the amount of transmit power available for packet data users. Second, the transmit power required by each scheduled terminal for a given data rate is dependent on the channel conditions and can also vary over time. Third, the terminals to be scheduled may have different scheduling delays (which may be defined, for example, as the difference between the time a terminal is scheduled and the time of actual data transmission). For example, some terminals may be in soft handoff and are supported by multiple base stations, and the scheduling delays of these terminals may be longer than those of terminals not in soft handoff.
There is therefore a need in the art for techniques to schedule data transmission to terminals with different scheduling delays.